Porphyromonas gingivalis is an important etiologic agent of periodontal disease. The harsh inflammatory conditions of the periodontal pocket, in addition to the environmental changes encountered during infection and colonization, suggest that this organism has properties that will facilitate its ability to respond and adapt to stress. Stress response in the pathogen is a major determinant of its virulence. In general, the response and adaptation mechanisms are known to be mostly regulated at the level of transcription initiation by extracytoplasmic function (ECF) sigma factor, the largest group of alternative sigma factors. To date, little is known about the relationship between the regulation of adaptive mechanisms, virulence, and ECF sigma factors in P. gingivalis. Because the hostile environment within the host will likely affect the membrane architecture of the invading bacteria, it is our hypothesis that in P. gingivalis ECF factors coordinately regulate mechanisms vital for protection against environmental stress and are significant in the pathogenicity of the organism. In the genome of Porphyromonas gingivalis W83, six putative ECF factors were identified. In preliminary studies, we have inactivated five ECF factor genes (PG0162, PG0214, PG0985, PG1660, and PG1827) by allelic exchange mutagenesis. Taken together, our findings suggest that in P. gingivalis ECF sigma factors can modulate important virulence factors. ECF factors encoded by the PG0162 and PG1660 genes might also be involved in the post-transcriptional regulation of the gingipains. Bioinformatics analysis suggests that PG0162 and PG1660 are paralogs that have unique properties. It is likely that they may be involved in unique and complex regulatory mechanisms. In this project, we wish to gain a comprehensive understanding of P. gingivalis ECF factors and their role in an adaptive response to the environmental conditions typical of the periodontal pocket. The Specific Aims are: (1) To confirm and characterize the properties of specific ECF sigma factors in P. gingivalis. (2) To identify the environmental stress signals and sensor protein(s) involved in the modulation of PG1660. (3) To identify the genes in the PG1660 regulon(s) and characterize the regulatory sequences involved in the expression of those genes. Collectively, the data generated will facilitate a comprehensive assessment of the role of ECF factors in regulating gene expression, identifying pathways of adaptation to environmental stress typical of the periodontal pocket and evaluating their impact on pathogenicity of P. gingivalis. Because many of the proteins in this study are unique, drugs designed to inhibit these targets could be an attractive therapeutic strategy to aid in the prevention of P. gingivalis-associated diseases.